It is generally accepted that during the extrusion of plastic, irregularities in the flow appear at the die outlet when a critical shear rate is exceeded. Below this rate, the extrudates are smooth, whereas, above this critical rate, surface defects are observed. These defects, which are referred to as "melt fracture", are in several forms. At a shear rate slightly above the critical rate, the films obtained by extrusion-blow moulding lose their transparency and their sheen. For markedly higher rates, which correspond to a higher production efficiency, homogeneity defects with smooth zones in a rough surface appear. These defects significantly decrease the optical and mechanical properties of the film. The same phenomena may be observed on extruded rods. When the surface of the rods loses its sheen and becomes dull and rough, it is often compared to an "orange skin".
Among the various solutions proposed to overcome this drawback, the use of modifiers is of greater value industrially. Thus, in U.S. Pat. No. 3,334,157, the incorporation of polytetrafluoroethylene improves the optical properties of the polyethylene film. According to U.S. Pat. Nos. 4,855,360 and 5,015,693, a fluoro elastomer in combination with a polyoxyalkylene is used to improve the conversion of hydrocarbon polymers. Organophosphates or organophosphites in combination with a fluoro elastomer have been described in U.S. Pat. Nos. 4,983,677 and 4,863,983 in order also to improve the conversion of hydrocarbon polymers.
However, additives such as anti-blocking agents, fillers, pigments, dyes and stearates, which are essential for the conversion of plastics, interact with these fluoro elastomers and significantly reduce their efficacy. To attempt to overcome this drawback, one process consisting in firstly treating the extruder die with modifiers and then in converting the plastics has been described in patent WO 91/05007. The modifier used contains a functionalized fluoro copolymer comprising tetrafluoroethylene units and a functional group containing a fluoro monomer. This process has the drawback of taking place in two steps.